Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/42—Selection of substances for use as reactor fuel
  • G21C3/58—Solid reactor fuel Pellets made of fissile material
  • G21C3/62—Ceramic fuel
  • G21C3/623—Oxide fuels
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E30/00—Energy generation of nuclear origin
  • Y02E30/30—Nuclear fission reactors
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S376/00—Induced nuclear reactions: processes, systems, and elements
  • Y10S376/90—Particular material or material shapes for fission reactors
  • Y10S376/901—Fuel

Definitions

• the present invention relates to nuclear combusti ⁇ bles based on UO2, IO2, and / or PuC> 2 having improved properties of retention of fission products.
• the combustion rate of nuclear fuel elements is currently limited to 50G j / t U so as not to exceed the threshold from which the release of fission gases becomes noticeable.
• additives such as TiC> 2, Nb2 ⁇ 5, Cr2 ⁇ 3, I2O3, V2O5 and MgO can be added to the uranium dioxide powder subjected to sintering to activate its crystal growth, provided that the sintering is carried out under a humid hydrogen atmosphere so that the amount of oxide added remains in solution in the uranium dioxide and is not reduced to a metallic element.
• the use of such additives to obtain a large grain microstructure is described for example by Killeen in Journal of Nuclear Materials, 88, 1980, p. 177-184; Sawbridge et al.
• Nanoprecipitates of this type can consist of magnesium oxide inclusions as described by Sawbridge et al., In Journal of Nuclear Materials, 95, 1980, p. 119-128 and in FR-A-2 026 251.
• another technique different from those described above is used to improve the retention rate of fission products in a nuclear fuel.
• the increase in diffusion coefficients is a mechanism which leads to the accumulation of fission products at the grain boundaries, then to the release of fission products.
• a reduction in the thermal conductivity of the fuel is harmful because it has the effect of increasing the temperature of the fuel for the same linear power and, consequently, on the one hand reducing the solubility fission products and, on the other hand, to promote their dissemination.
• the subject of the invention is a method for improving the retention of fission products within a ceramic nuclear fuel based on UO2, Th ⁇ 2 and / or PUO2, which consists in including in the ceramic nuclear fuel, minus a metal capable of trapping oxygen by forming an oxide having a free enthalpy of formation at the operating temperature T of the nuclear reactor lower than the free enthalpy of formation at the same temperature T, of the oxide (s) superstoichiometric (s) of formulas (U, Th) 0 2 + x and / or (U, Pu) 0 2 + x in which x is such that 0 ⁇ x ⁇ , 01.
• the free enthalpy of formation of the superstoichiometric oxide UO2 + X with 0 ⁇ x ⁇ 0.01 can be expressed in terms of potential of oxygen and calculated according to the law of Lindemer and Besmann, as described in Journal of Nuclear Materials, 130, 1985, p. 473-488.
• the oxygen potential ⁇ G ( ⁇ 2) of the superstoichiometric oxide defined above can be estimated in J / mol according to the following formula:
• metals capable of being suitable mention may be made of Cr, Mo, Ti, Nb and U.
• the invention also relates to a combustible material for a nuclear reactor which comprises a ceramic material based on UO2, Th ⁇ 2 and / or PUO2 in which at least one metal capable of trapping oxygen is dispersed, having the characteristics data above.
• the ceramic material based on oxide can consist of U0 2 F of Th0 2 r of Pu0 2 or their mixtures, of mixed oxide U0 -Pu0 2 or U0 2 -Th0, of mixed oxides with based on UO2 and other oxides such as rare earth oxides, or mixed oxides based on Pu ⁇ 2.
• the ceramic material is based on UO2 and the dispersed metal is capable of forming an oxide having an oxygen potential lower than the oxygen potential of UO2 + X as described above.
• the dispersed metal represents 0.1 to 2% by weight of the combustible material.
• the metal is chromium and it represents from 0.1 to 1% by weight, and better still from 0.2 to 0.5% by weight, of the combustible material.
• the combustible material can also comprise additives such as Ti ⁇ 2, Nb2 ⁇ 5, Cr 2 ° 3 ' A l2 ° 3' v 2 ° 5 e - M ⁇ ? 0, in order to increase the size of the fuel grains and / or to promote the anchoring of fission products, as well as other additives, for example Si ⁇ 2, to improve other properties.
• additives such as Ti ⁇ 2, Nb2 ⁇ 5, Cr 2 ° 3 ' A l2 ° 3' v 2 ° 5 e - M ⁇ ? 0, in order to increase the size of the fuel grains and / or to promote the anchoring of fission products, as well as other additives, for example Si ⁇ 2, to improve other properties.
• the combustible material of the invention can be prepared by conventional sintering methods by adding metal, either in metallic form or in the form of oxide or oxygenated compound, to the ceramic powder to be fried.
• sintering is carried out in a dry hydrogen atmosphere, for example having a
• REPLACEMENT (RULE 26) water content less than 0.05% by volume, so as not to oxidize the metal.
• the second case if one wants to obtain simultaneously a magnification of the grains of UO2, h ⁇ 2 and / or Pu ⁇ 2, an amount of oxide or oxygenated compound is used which may or may not exceed the solubility limit of the oxide or of the oxygenated compound in UO2, h ⁇ 2 and / or Pu ⁇ 2 at the sintering temperature, and after shaping the powder by cold compression, sintering is carried out under moist or humidified hydrogen, for example having a water content greater than 1 % by volume, to conserve the oxide during sintering and activate crystal growth. After sintering, the sintered material is subjected to a reduction treatment under dry hydrogen, for example having a water content of less than 0.05% by volume, to reduce the oxide or the oxygenated compound to metal.
• moist or humidified hydrogen for example having a water content greater than 1 % by volume
• the sintering is carried out under a hydro- dry gene, for example having a water content of less than 0.05% by volume, for simultaneously reducing the oxide or the oxygenated metal compound.
• micrometric intergranular metallic precipitates are obtained (diameter> 0.3 ⁇ m).
• the shaping of the powder by cold compression for example to form pellets, can be carried out conventionally by uniaxial compression, for example under pressures of 200 to 700 MPa.
• a temperature of 1750 to 750 ° C. is usually used.
• the powder of ceramic material comprising the metal additive in the form of oxide or of oxygenated compound can be prepared by mixing the powders of the constituents or also by spray-drying processes using a slip containing 1 additive in the form of a salt in solution, or by coprecipitation of a ura ⁇ nium salt and a salt of the additive.
• the process of the invention is therefore very interesting since it makes it possible to take advantage not only of the oxygen scavenging capacity of the added metal, but also of the properties of the metal oxides to activate the crystal growth of UO2 and improve retention of fission products within the fuel.
• FIGS 1, 2 and 3 are diagrams il ⁇ illustrating the evolution of the oxygen potentials of various oxygenated compounds (in kJ / mol) as a function of the temperature (in ° C),
• FIG. 4 is a micrograph of a combustible material according to the invention
• FIG. 5 is a micrograph illustrating the trapping of oxygen in a combustible material according to the invention
• FIG. 6 is a micrograph given for comparison to show the structure of a combustible material in accordance with the prior art after gentle oxidation
• - Figure 7 is a micrograph of a combustible material according to the invention having a small grain structure
• - Figure 8 is a micrograph of a combustible material according to the invention having a large grain mi ⁇ crostructure .
• the oxygen potential (in kJ / mol) is calculated, calculated according to the formula of Lindemer and Besmann for UO2 as well as for the superstoichiometric oxides UO2 + X and the substoichiometric oxides U ⁇ 2- x , depending on the temperature (° C).
• the evolution of the oxygen potentials (in kJ / mol) for the Cr / Cr 2 0 3 couple is represented as a function of the temperature (in ° C.) and it is noted that in the whole range of temperatures considered, the oxygen potential of this oxide is lower than that of the oxy ⁇ of the UO2 + X overstoichiometric of FIG. 1.
• these two elements may be suitable as a metal capable of trapping oxygen for materials.
• UO2-based fuel rials and the following examples illustrate the use of these two elements with UO2.
• a UO2 powder is used having an average particle size of 0.5 to 100 ⁇ m.
• Example 1
• UO2 pellets are prepared comprising micrometric metallic precipitates of Cr.
• UO2 powder 100 g are mixed by co-brewing with 0.1 g of metallic Cr powder with an average particle size of less than 2 ⁇ m, then the mixture is put in the form of pellets by uniaxial compression at 350 MPa with matrix lubrication in a hydraulic press. The pellets are then placed in a molybdenum basket and sintered at 1700 ° C for 4 hours under dry hydrogè ⁇ ne.
• FIG. 4 is a micrograph illustrating this structure at a magnification of 600. On this micrograph, a clear distinction is made between the presence of inter or intra granular metallic precipitates (white particles) and the electronic diffraction diagram confirms the metallic character of these inclusions.
• Figure 5 is a micrograph at a magnification of 400 which illustrates the structure of the combustible material having undergone this oxidation. In this figure, we see that the combustible material has trapped oxygen and does not include other phases than the UO2 matrix obtained previously.
• FIG. 6 shows the micrograph of a uranium oxide pellet obtained under the same conditions as those of Example 1, but without the addition of chromium, when the latter has was subjected to the same controlled oxidation to reach the average O / U ratio of 2.024.
• uranium dioxide nuclear fuel pellets are prepared having a microstructure with small grains of UO2 with micrometric metallic precipitates of Cr.
• 100g of UO2 powder is mixed by co-stirring with 0.15g of Cr2O3 powder (with a particle size of less than 2 ⁇ m), then pellets are formed from the mixture and sintered as in 1 ' Example 1, under an atmosphere of dry hydrogen.
• the added chromium oxide is reduced to metallic chromium during sintering under dry hydrogen, and it could not activate the crystal growth of UO2 to form a large grain microstructure.
• a small grain microstructure is thus obtained with metallic precipitates of Cr.
• Figure 7 shows this structure.
• a nuclear fuel is prepared having a microstructure with small grains of UO2 with metallic precipitates of Cr.
• a powder is prepared by spray-drying a slip containing 150g of UO2, 0.6g of a soluble chromium salt: (NH_j) 2Cr ⁇ 4 and 250g of distilled water.
• the powder obtained is then calcined for 2 hours in an alumina boat at 400 ° C. in a laboratory tube furnace in alumina under a stream of argon (300 ml / min) to transform the chromium salt into Cr 2 o 3
• a nuclear fuel is prepared having a coarse-grained microstructure of UO2 with metallic, nanometric and micrometric precipitates of Cr.
• a powder is prepared by spray drying as in Example 3, using 1.5 g of (NH4) 2Cr ⁇ 4, that is to say a Cr2O3 content greater than the solubility limit of Cr2 ⁇ 3 in UO2 at 1700 ° vs.
• the powder obtained is treated as in Example 3 by calcining it for 2 h in an alumina boat at 400 ° C. in a laboratory tube alumina oven under a flow of argon (300 ml / min), then it is placed under the form of pellets by uniaxial compression at 350 MPa as in Example 1.
• the sintering is then carried out under an atmosphere of hydrogen humidified with 1.7 vol% of water, at 1700 ° C.
• a re-cooked treatment is carried out at 1300 ° C., for 5 hours, under dry hydrogen, having a water content of less than 0.05% by volume to reduce the Cr 2 ⁇ 3 oxide to metallic chromium. Maintaining Cr 2 ⁇ 3 in the form of oxide during sintering has made it possible to use it as an activator of crystal growth and thus to obtain a coarse-grained micro ⁇ structure, and the annealing treatment under dry hydrogen has then allowed to reduce Cr2O3 to metallic chromium and thus to lead to nanometric and micrometric metallic precipitates.
• FIG. 8 The microstructure of the material obtained under these conditions is illustrated in FIG. 8 in which we note the large grains 1 of UO 2 , and the micrometric inclusions 5 of chromium. Nanometric chromium inclusions are demonstrated by electron diffraction. Example 5.
• a powder is prepared as in Example 3 by spray drying, but using 0.2 g of (NH4) 2Cr ⁇ 4, that is to say an equivalent content of Cr2 ⁇ 3 below the solubility limit of Cr2 ⁇ 3 in UO2 at 1700 ° C.
• the powder is then compressed in the form of pellets and sintered as in Example 4 to obtain a large grain microstructure due to the maintenance of chromium in the form of oxide.
• An annealing treatment is then carried out as in Example 4 to reduce Cr 2 O 3 to metallic chromium.
• Example 4 the same procedure is followed as in Example 4, but 1.5g of (NH4) 2Cr ⁇ 4 and 0.04g of ultrafine Si ⁇ 2 are used in the slip which contains 150g of UO2 and 250g of distilled water.
• the powder obtained by spray drying is compressed in the form of pellets, then sintered in a humidified hydrogen atmosphere and subjected to an annealing treatment under dry hydrogen, under the same conditions as those of Example 4. This gives a coarse-grained micro-structure of UO2 with metallic chromium precipitates and a silica phase at the grain boundaries.
• EXAMPLE 7 In this example, a mixture of 100 g of UO2 and 0.6 g of M0O3 is prepared by co-grinding, in a uranium metal ball jar, and then the powder mixture is compressed into tablets. sintered under the same conditions as those of Example 1. In this case, the molybdenum oxide is reduced to molybdenum during sintering and cannot activate the crystal growth of the UO2 grains. A microstructure with small grains of UO2 is therefore obtained with micrometric metallic precipitates of Mo.
• Example 8
• a powder is prepared by spray-drying an aqueous suspension composed of 150g of UO2 and 7.7g of ammonium heptamolybdate (NH4) 6 MB 7 ° 24, ⁇ 2 ° e - 250 g of distilled water . The powder is then treated as in Example 1.
• a small grain UO2 microstructure is thus obtained with micrometric metallic precipitates of Mo.

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• 1993
  • 1993-06-04 FR FR9306713A patent/FR2706066B1/fr not_active Expired - Lifetime
• 1994
  • 1994-05-25 TW TW083104760A patent/TW259871B/zh not_active IP Right Cessation
  • 1994-06-02 EP EP94917711A patent/EP0701734B1/de not_active Expired - Lifetime
  • 1994-06-02 JP JP7501394A patent/JPH09501491A/ja active Pending
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  • 1994-06-02 WO PCT/FR1994/000650 patent/WO1994029874A1/fr active IP Right Grant
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